(a) Field of the Invention
The present invention relates to a polyvinylchloride (PVC)-based impact modifier with superior anti-stress whitening properties. More particularly, this invention relates to a method for preparing a graft copolymer of methylmethacrylate-butadiene-styrene (MBS) having highly-balanced impact strength, transparency, anti-stress whitening, and processability, when it is applied to a polyvinylchloride resin composition.
(b) Description of the Related Art
Polyvinylchloride (PVC) is a polymer that contains more than 50% of vinylchloride, and thus it has a drawback of being brittle upon impact. In order to improve such a drawback of PVC resin, various methods have been studied heretofore. Improvements of impact resistance can be achieved, for example, by grafting monomers such as styrene, methylmethacrylate or acrylonitrile to a butadiene-based rubber latex. However, in this case, drawbacks exist in that transparency of the product is impaired and that stress-whitening can take place when it is molded into a sheet.
It is known that the properties of methylmethacrylate-butadiene-styrene (MBS) resin are significantly affected by the content of each of the monomers which form a graft polymer, the polymerizing method, and the content and particle size of the rubber latex which is used as a basic material. It is a general method to increase both the content and the particle size of rubber latex in order to improve impact strength, however, in this case, the transparency tends to be impaired due to the increase of the degree of scattering from the enlarged particle size of graft polymer particles dispersed in PVC resin. Moreover, when the refractive index of PVC resin and that of the graft polymer particle are quite different, or when deformation is carried out, micro-voids can be easily formed due to the weakening of binding forces between MBS resin and PVC resin, which causes the problem of an increase in stress whitening.
There has been much research undertaken with regard to the content and particle size of rubber particle, graft polymerizing methods, and compositions, in order to obtain graft copolymer particles having superior impact resistance and anti-stress whitening, and it is already known that when the usage of rubber latex in particular is restricted, transparency and impact strength of the product is significantly influenced by the monomer content which is subjected to grafting and the process.
Examples of prior arts include a process for improving anti-stress whitening and impact strength by controlling both a degree of swelling and the refractive index of rubber polymer (U.S. Pat. No. 4,431,772 to Katto et al.), by applying multi-step polymerization (U.S. Pat. No. 4,352,910 to Katto et al.), and by controlling monomers to be grafted stepwise (U.S. Pat. No. 4,857,592 to Hoshino et al.), and all these processes have in common the use of a cross-linking agent during graft polymerization. However, for all these attempts, the properties of PVC resin including grafted particles are restricted by the formation of protrusions such as un-dispersed fish-eyes, which are attributed to process parameters in connection with operation conditions and the like.